1. Field of the Invention
The present invention relates to a method of manufacturing a compound semiconductor integrated circuit and, more particularly, to a method of manufacturing a compound semiconductor integrated circuit obtained by continuously forming semiconductor layers having different compositions (i.e., different bandgaps) in the lateral direction.
2. Description of the Prior Art
Optical semiconductor integrated circuits on which functional elements such as semiconductor lasers, semiconductor light-receiving elements, and semiconductor modulators are integrated on a single substrate have been intensively studied and developed. This is because individual functional elements need not be connected to each other through passive waveguides such as optical fibers owing to integration to realize compactness, and the step of connecting passive waveguides and the like can be simplified to effectively reduce the cost.
As a conventional example of such optical integrated circuits, there is an optical switch with an integrated optical amplifier (Kirihara et al., OQD-90-61 printed by The Institute of Electrical Engineers of Japan) see also the tenth reference cited therein, which is T. Kirihara et al, in proc. top. meet. photonic switching 1993, pp. 25-28. In this report, the wavelength composition (wavelength corresponding to the forbidden width of a semiconductor material having a certain composition) of the material of an optical waveguide serving as an passive element is set to be shorter than that of an optical amplifier serving as an active element, thereby reducing an absorption loss. In this conventional example, however, to obtain different wavelength compositions of the passive waveguide and the optical amplifier, semiconductors having compositions corresponding to the respective wavelengths are formed in individual crystal growth steps, and crystals having different wavelength compositions are integrated on a single substrate.
The crystal growth step, therefore, must be repeated at least the same number of times as the number of functional elements having different wavelength compositions. The manufacturing yield of a device is degraded to undesirably increase the manufacturing cost, as compared with a case in which respective functional elements are independently formed.
As a conventional example to improve the above problem, there is an example to be described below (Kato et al., Electronics Letters Vol. 28 (1992), pp. 153). In this report, an SiO.sub.2 stripe mask is formed on an InP substrate in advance. InGaAsP is selectively, epitaxially grown in a non-masked region by metal organic vapor phase epitaxy (MOVPE) to integrate a distributed feedback laser (DFB-LD) and an electroabsorption optical modulator (EA-MOD).
This conventional example utilizes the principle that, as the stripe width of a selective MOVPE crystal growth mask increases, an In content in a non-masked region increases. The wavelength composition of InGaAsP is changed by changing the ratio of Group III elements (In and Ga in this example) in selectively grown crystals.
The above conventional example in which semiconductor layers having different wavelength compositions are epitaxially grown depending on the mask stripe width has the following problem because the composition difference is attained by changing the ratio of Group III elements.
A dashed line in the graph of FIG. 1 indicates the relationship between a wavelength composition change .DELTA..lambda. (nm) and a lattice strain .DELTA.a/a (%) when the ratio of Group III elements is changed at a constant ratio of Group V elements in a semiconductor layer formed by the above technique. In general, when a light-emitting element and a passive wave-guide are to be integrated, the wavelength composition of the passive waveguide must be shorter than that of the light-emitting element to avoid the absorption loss of light in the passive waveguide. As is indicated by the dashed line in FIG. 1, however, when the wavelength composition is changed by 100 nm or more by the conventional method, the lattice strain becomes 0.5% or more. Due to an excess lattice strain, a high-quality crystal cannot be obtained.